In radio communications terminology, VLF means Very Low Frequency, typically about 15 to 25 kHz.  VLF-EM is a geophysical survey technique that measures VLF signals from very powerful radio transmitters that induce electric currents in conductive bodies thousands of kilometres away.  These powerful transmitters are located around the world, and are used primarily for military communication purposes with submarines.  Signals from these transmitters act as primary fields that are capable of energizing conductive bodies (such as graphite, metallic minerals and structures) in the ground. Once energized, the current within these bodies emits a secondary field which can be measured by VLF-EM receivers.  VLF-EM surveys can be done with ground or airborne instruments, and data from these surveys can then be analyzed and interpreted to assist with mapping of conductive materials within the survey area.

 

VLF-EM Transmitter Stations

In North America there are four transmitters; a very powerful one in Cutler, Maine (24.0 kHz), another of medium power in LaMoure, North Dakota (25.2 kHz), one in Jim Creek, Washington (24.8), and another in Puerto Rico (40.8 kHz).  Around the world, additional VLF stations exist in Iceland, UK, Germany, Norway, Italy, Turkey, India, South Korea, Japan, Australia and Hawaii.   

Frequency (kHz)	Call Sign	Power	Location	Latitude	Longitude
16.3	VTX1		Southern India	+08.387015	+077.752762
16.4	JKN	45 kW	Novik, Norway	+66.974353	+013.873617
17.0	VTX2		Southern India	+08.387015	+077.752762
18.2	VTX3		Southern India	+08.387015	+077.752762
19.2	VTX4		Southern India	+08.387015	+077.752762
19.58	GBZ	500 kW	Anthorn, UK	+54.911643	-003.278456
19.8	NWC	1000 kW	Exmouth, Australia	-21.816328	+114.165586
20.27	ICV	20/43 kW	Tavolara, Italy	+40.923127	+009.731011
21.4	NPM	566 kW	Pearl Harbor, Hawaii	+21.420166	-158.151140
22.1	GQD	500 kW	Skelton, UK	+54.731799	-002.883033
22.2	NDT	200 kW	Ebino, Japan	+32.082084	+130.827960
23.4	DHO38	800 kW	Rhauderfehn, Germany	+53.078900	+007.615000
24.0	NAA	1000 kW	Cutler, Maine	+44.644936	-067.281639
24.8	NLK	250 kW	Jim Creek, Washington	+48.203487	-121.916827
25.0	unid25		Mokpo, South Korea	+34.679068	+126.445383
25.2	NML	500 kW	La Moure, North Dakota	+46.365990	-098.335638
26.7	TBB		Bafa, Turkey	+37.412725	+027.323342
37.5	NRK/TFK	100 kW	Grindavik, Iceland	+63.850365	-022.466773
40.8	NAU	100 kW	Aguada, Puerto Rico	+18.398762	-067.177599

 

 

Terraquest's Matrix VLF-EM system by Magenta Inc. is a newly developed digital frequency specific VLF system as well as broadband capability.  It utilizes three orthogonal coils mounted in the aircraft stinger, coupled with a receiver-console.  The primary signals originate from up to four VLF frequencies, and full parametrization of these VLF-EM signals are recorded.  The system typically responds to variations in overburden conductivity, to large faults or shear zones, and to graphitic formational conductors.  Because of these characteristics, Matrix VLF-EM can be useful as a geological structure mapping tool, particularly when combined with magnetics and radiometrics.

 

Terraquest has recently added optional inversion modelling to create resistivity products which further enhance the Matrix VLF-EM data to focus on project specific geological features.  Using these resistivity products, further interpretations from the VLF-EM data may include the following: